It would have been 9 ft. long and measured 8 stone in weight, equal to 112 lbs. and approximately 50.8 kg.
The creature, whose name is Arthropleura, roamed the coastal forests on hundreds of legs 326 million years ago, long before dinosaurs dominated the planet, back when Britain was having a tropical climate, and was positioned near the equator.
Students noticed the fossil of part of the creature’s exoskeleton, which was 2 feet 6 inches long, after a cliff slide in Howick beach, Northumberland, UK.
Experts used it to work out the full length. Arthropleura is the largest invertebrate in history.
Only two other examples have been found — both in Germany — but this one was older and bigger.
Dr Neil Davies, of Cambridge University, said: “It was an incredibly exciting find. The fossil is so large it took four of us to carry.”